In recent years, a demand for a larger backlight has been increasing as the screen of a liquid crystal TV set, etc., becomes bigger and bigger. To meet such a demand for a size increase plus demands for energy saving, lower prices, etc., a bent tube fluorescent lamp of a U shape or rotated U shape is fabricated by bending an elongated straight tube fluorescent lamp at its approximate center to replace a conventional straight tube fluorescent lamp with the bent tube fluorescent lamp, and a backlight incorporating such a bent tube fluorescent lamp as a light source is developed.
A backlight using the above bent tube fluorescent lamp achieves a relatively uniform temperature distribution when operates for a screen with a diagonal smaller than 17 inches, in which case a minor temperature difference or luminance difference in the direction of the tube axis arises but hardly poses a problem. As the screen size becomes further greater, however, lamp electrodes, which are a main heat source, come to have a higher temperature. In addition, voltage applied to lamps gets higher due to an increase in the length of the lamps, which produces a floating capacitance between a lamp and a unit, creating a condition where a leak current is generated easily near a high-voltage portion. Besides, a lamp power increase leads to an inverter power increase, which causes an inverter disposed on the back side of the lamp electrodes to generate a greater amount of heat. As a result, a temperature difference in the direction of the lamp tube axis grows greater.
The generation of an above leak current means that a lamp is kept turned on with a current value to the sum of a rated current and the leak current near the high-voltage portion in the direction of tube axis. This is the cause of a luminance difference arising in the direction of lamp tube axis. Hence a backlight having a greater size intensifies a tendency of an increase in a temperature difference and luminance difference in the direction of lamp tube axis, which causes the irregularities of luminance of the backlight.
In this manner, in a case of a backlight using bent tube fluorescent lamps (U shape, rotated U shape, etc.), the length of the fluorescent lamp increases with an increase in the size of the backlight, which leads to a higher applied voltage to the lamps. This results in the generation of a floating capacitance between a fluorescent lamp and an enclosure, etc., creating a condition where a leak current is generated easily near a high-voltage portion. As a result, a luminance difference arises in the direction of tube axis of the fluorescent lamps, becoming the cause of the irregularities of luminance of the backlight.
Besides, an increase in voltage applied to the fluorescent lamps leads to an increase in power supplied to an inverter driving the lamps, thus leading to an increase in heating value generated from the inverter. The inverter is disposed usually on the back side of the electrodes of the fluorescent lamps, so that a temperature difference in the direction of tube axis of the fluorescent lamps arises easily. Such a temperature difference too becomes the cause of a luminance difference in the direction of tube axis of the fluorescent lamps.
A solution to a problem of the irregularities of luminance of the backlight has been suggested. For example, Japanese Laid-Open Patent Publication No. 2005-268028 discloses a backlight that is capable of illuminating with uniform luminance even when illuminating a large-screen liquid crystal display.
According to Japanese Laid-Open Patent Publication No. 2005-268028, the electrode side and bent side of bent tube fluorescent lamps are arranged alternately, and each inverter is disposed right on the back side of a reflective case (enclosure) at the electrode power supply side of each fluorescent lamp, so that a tube wall temperature difference in the direction of the lamp tube axis and of the panel surface is reduced and a luminance difference on the panel surface is also reduced.
According to the invention described in Japanese Laid-Open Patent Publication No. 2005-268028, when a worker tries to arrange U-shaped lamp units in the reflective case, etc., in orderly fashion in mounting the lamp units on the backlight, the worker must put the lamp units in such an arrangement that the second lamp electrodes are located opposite to the first lamp electrodes. This arrangement work often involves work of counting lamp units to be mounted and of shifting the lamp unit from one hand to the other upon mounting the lamp units, especially upon mounting fluorescent lamps for use in the backlight, which fluorescent lamp has a diameter of about two to five millimeters and a length of about several tens of centimeters to several tens of meters. This makes mounting of the lamp units troublesome, and may invite problems of erroneous mounting, lamp breakage, etc.
According to the above backlight disclosed in Japanese Laid-Open Patent Publication No. 2005-268028, the U-shaped fluorescent lamps must be so arranged that the electrode side and the bent side of the fluorescent lamps are set alternately in rows when the fluorescent lamps are housed in the reflective case. This necessitates repeating work of locating the second lamp electrodes opposite to the first lamp electrodes and arranging the next lamp electrodes in the same manner as the first electrodes.
This case involves work of counting fluorescent lamps to be mounted and of shifting the fluorescent lamp from one hand to the other, which makes mounting of the fluorescent lamps troublesome and may invite problems of erroneous mounting and breakage of the fluorescent lamp. Such problems are easy to occur, particularly, in mounting a fluorescent lamp for use in the backlight, which fluorescent lamp has a diameter of about two to five millimeters and a length of about several tens of centimeters to several tens of meters.
The fluorescent lamps to be mounted on the backlight are usually packed in a packaging material, such as a cardboard and are distributed to be supplied to a backlight production process. In this case, the U-shaped fluorescent lamps are arranged horizontally in the packaging material, where the fluorescent lamps are stacked in tiers via buffering materials, etc.
On a production line for the backlight whose size further increases as a screen size further increases, an amount of fluorescent lamps to be used on the production line becomes enormous, which results in an increase in the capacity of a packaging material containing the fluorescent lamps. This leads to problems of an inefficient production process, an increase in a space occupied by the fluorescent lamps around the production line, etc.